Keeping light and cool with smart windows

Indoor natural lighting often comes with a sun-baked room. But with a new window coating, light shines through while heat gets denied entry.

Letting the sun light up rooms and offices cuts lighting costs and boosts moods. But air-conditioning costs climb, too, as sunshine radiates through windows.

A hot ticket to energy savings all around would be glass that customizes indoor environments to our visual and temperature preferences. Those can change with the time of day or season, but adjustable windows that give us more of what we want have been emerging. Government researchers are now trying to fine-tune these smart windows with a thin film of nanocrystals.

Windows can be smart in more ways than one. Some generate power by incorporating solar cells . Others feature self-tinting glass that can welcome heat and light in or block them both out. But getting one without the other is a problem with the latter. Researchers from the Lawrence Berkeley National Laboratory are getting closer to changing that. They developing a window coating that lets light shine through while it helps keeps heat out.

With a flip of a switch, the semiconducting material keeps the window clear but limits the amount of near infrared light passing through it. These wavelengths contribute to the light heating up a room but they’re invisible—so our eyes don’t really notice their absence. Publishing recently in NanoLetters, the researcher team reports that when switched on, the coating could transmit 35 percent less near infrared light.

Delia Milliron of the lab’s Molecular Foundry says in a statement:

To have a transparent electrochromic material that can change its transmittance in the infrared portion of sunlight is completely unprecedented. What’s more, the coloration efficiency of our material—a figure of merit describing the amount of current needed to make this thing go—is substantially higher than standard electrochromic materials, which means it’s also very efficient.

Should the new coating eventually prove durable enough and cost-effective for covering many windows with it, the energy efficiency wouldn't stop there. About a third of the energy consumed for heating and cooling buildings goes out the window (a traditional window, that is). Smart windows coatings, according to the Department of Energy, could potentially slash lighting and air-conditioning usages in half.

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Image: Flickr/bluemoondream

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